Shoot a couple test pics before everyone gathers and adjust the camera accordingly and have it all set for them.
Based on your tests, set the camera to shoot at the fastest shutter speed possible. Test again.
Keep the camera zoomed out. To frame the shot, move the camera closer to the people rather than zooming in. This will allow you to set the camera at f2.0 (more light) and reduce magnifying the shake (when you zoom in, not only do you magnify the scene, you magnify the effects of camera shake).
Set the manual focus to eliminate shutter lag.
Have the person look through the eyepiece rather than through the LCD with outstretched arms. (In fact, close the LCD to eliminate the temptation.) A three point stance (two hands and a head) is much more steady than outstretched arms.
Use the two second timer to eliminate camera movement when pressing the shutter. Inform them what you are doing so they know to keep the camera on target until if fires.
Dope them up with some 'ludes.
If possible, bring a bean bag (or three hacky sacks) and set the camera on that instead of giving it to Aunt Marge. Unless you don't want her in the picture anyways.
Good luck.